Conventionally, as a typical synthetic method of a fine grain of polymer, a microemulsion polymerization (polymerization in microemulsions) method has been known. One example of the polymerization method is a synthetic method of a fine grain of polymer having a particle diameter of 10 to 100 nm or so, comprising solubilizing an unsaturated monomer (oil phase) in water phase under the presence of the large amount of an emulsifier and then, polymerizing the unsaturated monomer (cf. Macromol. Chem. Phy., 196, p.441-466 (1995)). Because various functions can be imparted to a fine grain of polymer obtained by the polymerization method by introducing functional groups into the surface of the fine grain, there are expectations of practical use of the fine grain in various uses.
When an aqueous dispersion of the fine grain of polymer having a particle diameter of at most 100 nm is prepared by the microemulsion polymerization method, as mentioned above, the large amount of the emulsifier is required. Accordingly, the obtained aqueous dispersion of the fine grain of polymer cannot always satisfactorily show needed properties because the aqueous dispersion strongly foams during using and water resistance of a film obtained from the fine grain of polymer is lowered, owing to the large amount of the emulsifier. Now, in order to overcome these defects, it is examined to use a high molecular emulsifier. However, because emulsifying force of the high molecular emulsifier is smaller than that of a low molecular emulsifier, there are generated other defects such that an emulsion is broken during polymerization, aggregates are generated, particle diameter of the resulting fine grain of polymer becomes larger with the passage of time and storage stability is lowered.
The present invention has been accomplished in consideration of the above prior art, and aims at providing an aqueous dispersion of a fine grain gel of polymer showing excellent polymerization stability, storage stability and water resistance and less susceptibility to bubbling, and a process for preparing the same.
In order to solve the problems in the above prior art, the inventor of the present invention has earnestly studied. As a result, the inventor has found that a specific cationic polymer containing a quaternary ammonium group shows great effect for solubilizing unsaturated monomers such as styrene and acrylic acid ester, and has excellent stability during microemulsion polymerization (polymerization in microemulsions). Also, the inventor has found that by introducing a polymerizable double bond into the side chain of the above cationic polymer, the cationic polymer is polymerized with the unsaturated monomer and a crosslinked structure can be imparted to the resulting fine grain. The present invention has been accomplished based on this knowledge.